XJ750 starter cranks but no start.

Hi Guys,
1981 Xj750 Carbs are clean, I have a good battery(my Lipo 14.7),solenoid is new, and ea plug appears to be getting spark. She refuses to turn over.
I dont think I have to pull in the clutch lever or raise the sidestand to enable a start(but I do these things Anyway).

I have pulled plugs individ to see if the cylinder has compression, they ea seem to have a forced "pfft!". (i will buy real test kit)
If I press starter button WITH start fluid into carb 1 OR 2 she coughs, but will not run. Starting with fluid into carb 3 causes a racing engine, which needs to be shutoff, carb 4 I didnt bother, to diff to get hand in there. (I have stock airbox which makes it nearly impossible to spray fluid into any carb.)

I cleaned the choke supply,tubes,lifters caps,spring,pins etc... those worked fine before anyway.

This bike started fine last fall...one press of starter button with half choke.

If it coughs on fluid then it cant be electrical...imo.

I took the tank off to use a valveless,temp gas supply. I know my floats are closing.
I pour just enough gas, so no more than 3 inch of gas in the line before it enters the main shaft to bowls.

Im baffled.

 

My guess is it’s carby related. Probably not what you want to hear, but if it was me I’d pull the carbides apart and check all slides are free, all ports and jets clear etc. also worth checking the rubber intake boots…
You could also fit a clear tube from the base of each bowl to check the actual fuel level is right.

 

I checked slides, all seem to come down at same rate... not snappy like a guillotine, but come down with a bit of resistance. Not sure if the slight bit of Marvels Mystery oil I put on the slides hurts em.

3 yrs ago I replaced the rubber on the diaphrams.These required you cut the plastic ring off and,dont have the little tab. I used gasket sealer to make sure no air passed by the rubber contacting the slide. The engine ran fine with these for last 2 yrs.

 

Blocked float bowl enrichment jets for cold starting?

 

You can snip off a small stretch of clear tubing and open up the float bowl drains to see how the bowls are filling.
Below are pics of me doing just that showing how high up the fuel should go (I'm on center stand). This is a ROUGH measurement. if you get none or say half that, bowls are not filling. You set floats on flat surface 3mm from top of bowl, but if you notice, I drew a line showing how that line meets up with about 3mm if your looking at the center of the bowl from the side where the main jet is. That's why main jet is in the middle and why fuel level remains the same even when bike is on a lean.


pull seat off and cover of air filter. Squirt some carb start past the filter across the carb intakes. Then crank to see if you get a few seconds of run. If your fuel enrichment is the only thing blocked, that should be enough to start it too.

 

I pulled the bowls, got in there with small drill bit for angled hole that is exposed to the inside of bowl( light twisting with fingers to release any crap), then move to Guitar wire at top edge of bowl into choke tube receiver to clear the tiny hole at base of bowl. Pin lit hole...check. i also blasted the tube itself , shooting carb fluid upward into the tube, and the fluid came out over the top of the butterfly. Clear I think?

When i spray carb fluid into the rear airbox boots...I get a
Coughing start that lasts till the fluid gas is gone.
My compression on all cylinders is 150+,and I can see each valve moves freely.
I have spark...cause if it coughs with Cfluid, the spark for gasoline should be ok, no?

It sounds to me like its just not getting gas. No amount of choke ,or no choke has any impact.I checked those pins and caps too.
Another fella suggested my carbs werent seated into boots fully....but pushing em in is tough, but when they go in ...they are "in".

 

What voltage are you getting at the battery terminals when you press the starter button? You need minimum voltage of 10 volts for the transistors in the TCI to work. You have 14.7 volts what is it under cranking when the starter motor is robbing the TCI of the necessary minimum voltage. It might be fine but worth checking. I know you have spark but still worth checking the battery.

 

Franz...haah that was my pseudonym in high school German class...anyway...

I have 2 batteries which I now swap for all of the random "cranking" I'm doing as I troubleshoot. I have 1 new AGM battery with 250 CCA, a LIPO battery with 400+CCA...when fully charged it registers 14.7(it also weighs 2lbs!.
I will check the voltage using my meter on pos and the neg terminal, and start it to see what I get. KIM...the bike would try to start on Carb fluid(being more flammable)but just cranks when I hook up the temp gas line, and choke it normally to start....
Compression test yesterday shows 150+ on all cylinders. This thing used to turn over with slight choke and 1 or two presses of the start button. I like the Lipo battery cause it cranks it over quickly. The starter has been rebuilt, and solenoid is new too. I did not pull my mixture screws. I did not break the carbs..just cleaned bowls,air jets, pilot jet, choke lines. I see spark on ea plug...
She is either starving for gas or the spark is weak to ignite the gas....imo.

Where should I look for the 10volts...put tester on Battery terminals ...or place them somewhere else?

 

On the battery terminals. Spark should be good with the battery you have assuming of course the required voltage is getting to the red wires with white stripe at the primary side of your ignition coils.

 

At battery, meter says 12 .5 when i press start button.

 

I just bought new coils from Len.
Do I have to cut off the relay bracket on left side or is it ok to run the #1 wire Over the bracket